1. Field of the Invention
The present invention relates in general to an electrophotographic copying machine. More particularly, it relates to an electrophotographic member for electrophotographic copying machines and a method of forming an electrophotographic member for electrophotographic copying machines.
2. Description of the Prior Art
An electrophotographic copying machine uses photoconductive effects to form the image. An image of the document to be duplicated is reproduced on a photosensitive surface in the form of toner patterns by virtue of electrostatic forces. In the CPC mode, the toner pattern is directly formed on a photosensitive sheet, which is used as a copy. In the PPC mode, the toner pattern is transferred from a photosensitive surface to a usual paper. The present invention relates mainly to the PPC mode. Xerography, also called Carson process, is a representative process of the PPC mode. The process comprises charging, exposure, development, transfer, fusing and cleaning. The process is implemented by a photosensitive printing member (drum), an electric charging device, an exposing device, a transferring device, a fusing device, an electricity remover and a cleaner.
The life time of such an electrophotographic copying machine is primarily determined by the life time of the photosensitive member (drum). Namely, the reliability of an electrophotographic copying machine is significantly improved by improving that of the photosensitive member (drum).
Photosensitive materials coated on the drums include organic resins, amorphous silicons and selenium alloys. The surfaces of such photosensitive coatings tend to experience abrasion caused by frictional forces at the contact with a transfer sheet, development roller and a cleaning blade or brush. The wearing of the coating is further accelerated by toner or a lubricant contained in toner. For this reason, some attempts have been tried to coat the surface with protective films made of organic resin, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4 and other inorganic materials.
The resistivity of such protective films has to be between 10.sup.8 and 10.sup.12 .OMEGA.cm. If the resistivity is less than 10.sup.8 .OMEGA.cm, diffusion of electric latent images, i.e. lateral leakage of charge takes place resulting in degraded images out of focus. If the resistivity is higher than 10.sup.12 .OMEGA.cm, electric charge is always accumulated in the film and hindered normal dissipation of electric charge and clear formation of images during repetition of copying. Namely, the quality of the copied images can not be maintained for a long time. The transmissivity of the protective film, on the other hand, has to be no lower than at least 50% in a range between 400 nm and 800 nm, and not lower than 80% in a range between 500 nm and 800 nm, taking into account the application to color copy.
The applicant filed a Japanese patent application No. sho 56-146936 on May 17, 1981 titled "Complex with a carbon film and a Method of Manufacturing the Same" to propose protective films which satisfy abovementioned requirements such as abrasion-proof, necessary optical and electrical characteristics. Films mainly composed of carbon can be easily formed to be substantially transparent to visible light with Vickers hardness higher than 2000 Kg/mm.sup.2. The resistivity can be controlled between 10.sup.8 and 10.sup.13 .OMEGA.cm by adjusting the deposition condition of the carbon film. This is convenient for application to photosensitive films for copying machines.
The carbon films have good compatibility and adhesivity with organic photoconductive materials (OPC). These desirable properties are considered because there may be formed polymerization at the interface between the carbon film and the OPC film. It has been confirmed by the inventor, however, that there has to be a trade-off among several important characteristics of the carbon film. Namely, whereas Vickers hardness higher than 2000 kg/mm.sup.2 is easily obtained, the absorption coefficient eventually rises, for example, to 0.6 .mu.m.sup.-1, which corresponds to a transmissivity of 74% in the case of 5000 .ANG. thickness, and the resistivity is lowered to 10.sup.6 to 10.sup.7 .OMEGA.cm. This trade-off results in low sensitivity due to weak light passing through the protective film and disturbance of image formation due to the low resistivity. Contrary to this, low absorption coefficient as low as 0.2 .mu.m.sup.-1 can be obtained at the expense of Vickers hardness as low as 500 kg/mm.sup.2 and resistivity as high as 10.sup.14 to 10.sup.15 .OMEGA.cm.